The study of the relationships between physico-mechanical behavior and microstructure of metals has progressed over the past 50 years to the point where the microstructural basis for many properties of alloys is well understood. Microstructure is now a major factor in alloy design. Advances in the study of brittle, nonmetallic materials over the same period have not been as rapid, suffering mainly from the difficulty in sample preparation for microscopic examination. This difficulty, coupled with the relatively recent interest in porcelain as a dental restorative material, account for the dearth in published material on dental porcelain microstructure. Increasing demands are being made on dental porcelain as a restorative material due to the trend toward full procelain coverage in order to reduce precious metal costs in esthetic crown-and-bridge practice. A major factor influencing the success of a porcelain-fused-to-metal restoration is the degree of thermal expansion mismatch between porcelain and metal and between different layers of porcelain. A lack of information on porcelain microstructures has hindered the interpretation of data collected on porcelain-metal thermal compatibility. In the past decade, a technique has become available for preparing ultrathin polished sections of brittle materials for transmitted light microscopy. This technique, originally developed at Battelle Pacific Northwest Laboratories for examining the lunar rocks, would allow the visualization of dental porcelain microstructures by minimizing overlap of the phases and crystalline inclusions. The objective of this proposed research is to adapt the battelle technique to prepare ultrathin samples of dental porcelain, and to discern possible relationships between the microstructure thus revealed and the observed thermal expansion behavior. An understanding of the microstructural basis of dental porcelain behavior would allow us to discern the cause of failure of a given porcelain system as well as to design systems more resistant to failure.